(a) Field of the Invention
The present invention relates to a display device having a plurality of leads connected to a single common line, and more particularly, to a display device having a display area and a peripheral area, wherein drive ICs for driving signal lines and power supply lines disposed in the display area are coupled to the leads in the peripheral area.
(b) Description of the Related Art
The LCD device generally includes a pair of glass substrates opposing each other to sandwich therebetween a liquid crystal (LC) layer, the LC layer being applied with drive voltages for representing thereon desired images. FIG. 7 shows a sectional view of a typical LCD device connected to drive ICs (not shown) mounted on tape carrier packages (TCPs). The LCD device 200 includes first and second substrates 201 and 202 sandwiching therebetween an LC layer 205, wherein the first substrate 201 is larger than the second substrate 202. For example, the first substrate 201 mounts thereon active elements, power source lines, data lines and scanning lines, whereas the second substrate 202 mounts thereon color filters. The TCPs 203 mounting thereon drive ICs are coupled to the first substrate 201 via terminals (peripheral terminals) 204 disposed in the peripheral area 206, at which the first substrate 201 protrudes from the second substrate 202. The second substrate 202 opposes the display area 207 of the first substrate 201.
FIGS. 8A and 8B show details of alternative configurations in the vicinity of the peripheral terminals 204 in the peripheral area 206 shown in FIG. 7. The leads of data lines, scanning lines and power supply lines extending in the display area 207 of the first substrate 201 are configured by ITO (indium-tin oxide) lines 211 shown in FIG. 8A or metallic lines 214 shown in FIG. 8B, and extend from the display area 207 to the peripheral area 206.
These ITO lines 211 or metallic lines 214 are protected by an insulating film 212 and exposed from openings of the insulating film 212, at which ITO pads 213 are formed for connecting the ITO lines 211 or metallic lines 214 to the TCPs 203. The ITO pads 213 are connected to electrodes 210 of the TCPs 203 via anisotropic conductive film wherein conductive particles are dispersed in a resin, and are covered by a resin film (not shown). A portion of the peripheral area 206 receiving therein the peripheral terminals 204 including the ITO pads 213 is referred to as a driver mounting area 208.
In the structure shown in FIG. 8A, since the ITO line 211 has a higher line resistance compared to the metallic line 214 shown in FIG. 8B, there is the problem of a lower speed operation of the display device having the ITO line 211. On the other hand, in the structure shown in FIG. 8B, the metallic line 214 is susceptible to erosion caused by ingress of water or disconnection caused by electro-migration. In short, there is a trade-off between the structures shown in FIGS. 8A and 8B as to a lower line resistance for achieving a higher operational speed and a higher erosion resistance for achieving a higher long-term reliability of the display device.
Patent Publication JP-A-2003-98543 describes a technique for solving the above trade-off by using the structure shown in FIG. 9, which shows the vicinity of the peripheral terminals 204 similarly to FIGS. 8A and 8B. The drive IC in the publication is mounted on the first substrate by using a COG (chip on glass) technique, wherein each lead is configured by a first-layer line 221 made of tantalum, a second-layer line 222 made of chrome and a third-layer line 223 made of ITO. Although the second-layer line 222 made of chrome has a lower erosion resistance compared to the tantalum and ITO, the second-layer line 222 has a lower electric resistance and provided only in the driver mounting area 208. The second-layer line 222 reduces the line resistance of the display device while being protected against the erosion due to the structure sandwiched between the first-layer line 211 and the third-layer line 213.
It is noted in the structure of the LCD device that the peripheral terminals include one or more common terminal sets in addition to data terminals and scanning terminals connected to data lines and scanning lines corresponding to respective rows and columns. The common terminal set includes a plurality of common terminals connected in common in the display area 207 and generally connected to all the rows or columns, and thus is requested to have a lower electric resistance and a higher current capacity. In this respect, a plurality of common lines connected in common in the display area 207 are generally connected via respective leads to respective common terminals of a common terminal set, and connected to the drive IC via respective leads.
FIG. 10 schematically shows an example of the typical LCD device. The LCD device includes a LCD panel configured by the substrates 201 and 202 and defining a plurality of pixels 209 therein, two TCPs each configuring a Y-driver 203 for driving scanning lines of the pixels 209, and three TCPs each configuring an X-driver 203 for driving data lines of the pixels 209. In the exemplified configuration, each Y-driver or each X-driver 203 is connected to a pair of common line sets 215 each including three leads connected in common in the display area 207.
The peripheral area 206 of the LCD device 200 shown in FIG. 10 can be implemented by the structure shown in FIG. 9 having a three-layer lead. However, if the LCD device operates for a long term, the structure shown in FIG. 9 is also susceptible to erosion of the second-layer line 222 due to the ingress of a small amount of water through the driver mounting area 208. Thus, the structure shown in FIG. 9 may eventually have a larger line resistance in the common leads, to degrade the image quality or cause a malfunction in the image display. This problem is common to another type of display device using a drive IC directly connected to the substrate.